1. Field of the Invention
This invention is directed to drive assemblies for self-propelled wheeled devices.
2. Related Art
Devices or machines having self-propelled wheels typically occupy two distinct points along the size spectrum. At one end of the side spectrum lie the “mega machines” or “monster machines”, as they are commonly referred to in the media. These large-scale machines are typically used to move extremely heavy and/or large loads, and thus typically have extremely large or oversized wheels and/or tires that are typically in the six to twelve foot diameter range. Such large-scale machines are disclosed in, for example, U.S. Pat. Nos. 3,197,229; 3,280,931; 3,645,406; 4,003,447; 4,599,030; 5,165,838; 5,618,151 and 6,328,123 and published U.S. patent application Ser. No. 2002/0175009.
Because the wheels for such large-scale machines are so large, it is possible to place electromotive, hydraulic, pneumatic or other large-scale drive mechanisms directly within the hub of such wheels. Typically, at least some, if not all, of the wheels of such large-scale machines will have independent, direct-drive motors. Due to the size of such wheels, such motors may incorporate any necessary reduction gearing in the assembly provided within the hub of the wheels. Typically, such large-scale self-propelled wheels provided only limited steering ranges, although some of the more complicated designs, such as that shown in the '030 patent, will allow for 360° rotation of the wheels.
However, due to the complicated nature of such drive systems, it is effectively impossible to scale such drive systems down for use with small-scale self-propelled wheels. Additionally, because the power/weight ratios of such large-scale drive systems do not adequately scale down, even if it were possible to provide a scaled-down version of the drive system, the amount of power that could be obtained from such scaled-down large-scale drive systems would be inadequate for small-scale wheels.
At the other end of the size spectrum lie small-scale devices or machines having self-propelled wheels, such as wheelchairs, self-propelled material pallets and platforms, mobile robots, robotic carts and the like. Such small-scale devices are disclosed, for example, in U.S. Pat Nos. 4,573,548; 4,657,104; 5,609,216 and U.S. Pat. No. 6,853,877 and in “Development of a holonomic mobile robot for mobile manipulation tasks”, by R. Holmberg et al., FSR'99 International Conference on Field and Service Robots, Pittsburgh, Pa., August 1999, pp 1-6. Each of the '877 patent and Holmberg is incorporated herein by reference in its entirety.
Such small-scale machines typically use a simple two-wheel drive system, where each wheel has its own motor and the machine is balanced on one or more caster wheels in addition to the two driven wheels. This simple design has significant limitations, such as poor or imprecise position, velocity and/or acceleration control, and the inability to move freely in any direction. For example, typical wheelchairs cannot move sideways. Such small-scale machines are also subject to rocking about the caster wheels, which typically results in loss of traction and vehicle sway. Additionally, the pivot point of the driven wheel on the supporting surface is often collinear with the steering axis and the pivot or contact point of the wheel is often quite small. Because such small-scale self-propelled machines typically support significant loads relative to the size of the contact area, rotating the wheels about the steering axis/pivot point typically causes the wheels to slide, rather than roll on the support surface, such as a floor, thus scuffing the support surface. Scuffing not only can leave marks or other discolorations on the support surface, but can actively damage the support surface if the sliding friction of the wheel is sufficiently high.
Some small-scale self-propelled machines use a synchronous drive train, which is also know in this art as a “synchro-drive”. Small-scale self-propelled machines that use a synchro-drive system typically have a minimum of three driven wheel assemblies and all of the wheel assemblies are steered and driven in unison, i.e., synchronously. That is, each of the driven wheel assemblies receive the same drive signal indicating forward or backward motion and speed and the same steering signal which indicates direction relative to a reference point on the small-scale self-propelled machine. These signals can be either electronically or mechanically transmitted to the wheel assemblies. It should be appreciated that at least three wheels or wheel assemblies are used because three points are necessary to define a plane and thus the minimum number of wheels to define a meta-stable small-scale machine.
When such machine or devices that incorporate synchro-drive systems operate, the machine or device does not rotate as the wheels pivot for steering. Accordingly, such synchro-drive machines remain in the same orientation regardless of their direction of movement. Straight-line motion is improved, because steering drift is minimized by the locked geometry of the gearing. However, locked gearing also prevents many precision movements. Since the wheels cannot move independently, such synchro-drive machines cannot pivot, spin on their axis, or perform a simulated skid as a maneuver. Additionally, large volumes of space are taken up by the centralized motors, gears, belts, chains, shafts and other mechanical devices used to transmit power from the central motors synchronously to the driven wheel assemblies.
In contrast to synchro-drive machines, small-scale self-propelled holonomic machines use two or more self-propelled wheel assemblies, where the overall holonomic drive system has at least one motor for each degree of freedom. Typically, such holonomic systems use independent steering and drive motors for each wheel assembly. The drive assemblies for such holonomic systems typically have three degrees of freedom for motion within a plane. As discussed in the incorporated '877 patent and Holmberg, on very small machines, small, caster-type wheels are often used, while more standard or traditional sized holonomic machines use standard round wheels. In either case, the weight distribution of the supported load on each wheel assembly is an issue, as all the weight for each wheel is concentrated onto the small surface area contact patch of the wheel. Again, rotation around the steering axis, whether in synchro-drive machines or holonomic machines, typically causes scuffing and floor damage.
It should also be appreciated that such small-scale self-propelled machines, whether synchro-drive machines or holonomic machines, typically omit any suspension elements and are unable to adjust the height of the support platform relative to the wheel assembly.